Such flow measuring devices operate on the basis that vortices are shed alternately from both sides of a blockage, around which a fluid is flowing in a pipeline, such that a so called Karman vortex street forms, wherein the vortices lead to periodic pressure fluctuations, which are registered with pressure sensors or a paddle. The shedding frequency of the vortices for a particular measuring arrangement is proportional to the flow velocity and to the Strouhal number, a dimensionless number, which has a weak dependence on the Reynolds number. For wide ranges of Reynolds numbers, the Strouhal number can to a first approximation be assumed to be constant. Especially for Reynolds numbers under 20000, however, this no longer holds, so that the model ascertaining the flow based on the vortex frequency must be refined.
An approach to this is described in European Patent, EP 0 619 473 A1, according to which the pressure fluctuations are registered with two paddles spaced differently far from the blockage. The phase difference between the registered pressure fluctuations is then taken into consideration for correcting the flow measurement. The phase difference has, however, large fluctuations, so that the signal processing for ascertaining it is very complex. This is true especially when a compact flow measuring device is desired, in the case of which the locations for registering the pressure fluctuations are arranged near the blockage and not spaced far from one another.